Transition metal/zeolite scr catalysts

ABSTRACT

A method of converting nitrogen oxides in a gas to nitrogen by contacting the nitrogen oxides with a nitrogenous reducing agent in the presence of a zeolite catalyst containing at least one transition metal, wherein the zeolite is a small pore zeolite containing a maximum ring size of eight tetrahedral atoms, wherein the at least one transition metal is selected from the group consisting of Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag, In, Sn, Re, Jr and Pt.

This application is a continuation of U.S. application Ser. No.13/164,150, filed on Jun. 20, 2011, which is a continuation of U.S.application Ser. No. 12/597,707, filed on May 7, 2010, which is a 371 ofPCT/GB2008/001451, filed on Apr. 24, 2008, which is turn claims priorityto PCT/GB2007/050216, filed on Apr. 26, 2007, the entire contents ofwhich are fully incorporated herein by reference.

The present invention relates to a method of converting nitrogen oxidesin a gas, such as an exhaust gas of a vehicular lean-burn internalcombustion engine, to nitrogen by contacting the nitrogen oxides with anitrogenous reducing agent in the presence of a transitionmetal-containing zeolite catalyst.

Selective catalytic reduction (SCR) of NO by nitrogenous compounds, suchas ammonia or urea, was first developed for treating industrialstationary applications. SCR technology was first used in thermal powerplants in Japan in the late 1970s, and has seen widespread applicationin Europe since the mid-1980s. In the USA, SCR systems were introducedfor gas turbines in the 1990s and have been used more recently incoal-fired powerplants. In addition to coal-fired cogeneration plantsand gas turbines, SCR applications include plant and refinery heatersand boilers in the chemical processing industry, furnaces, coke ovens,municipal waste plants and incinerators. More recently, NO reductionsystems based on SCR technology are being developed for a number ofvehicular (mobile) applications in Europe, Japan, and the USA, e.g. fortreating diesel exhaust gas.

Several chemical reactions occur in an NH₃ SCR system, all of whichrepresent desirable reactions that reduce NO to nitrogen. The dominantreaction is represented by reaction (1).

4NO+4NH₃+O₂→4N₂+6H₂O  (1)

Competing, non-selective reactions with oxygen can produce secondaryemissions or may unproductively consume ammonia. One such non-selectivereaction is the complete oxidation of ammonia, shown in reaction (2).

4NH₃+5O₂→4NO+6H₂O  (2)

Also, side reactions may lead to undesirable products such as N₂O, asrepresented by reaction (3).

4NH₃+5NO+3O₂→4N₂O+6H₂O  (3)

Aluminosilicate zeolites are used as catalysts for SCR of NO with NH₃.One application is to control NO emissions from vehicular dieselengines, with the reductant obtainable from an ammonia precursor such asurea or by injecting ammonia per se. To promote the catalytic activity,transition metals are incorporated into the aluminosilicate zeolites.The most commonly tested transition metal zeolites are Cu/ZSM-5,Cu/Beta, Fe/ZSM-5 and Fe/Beta because they have a relatively widetemperature activity window. In general, Cu-based zeolite catalysts showbetter low temperature NO reduction activity than Fe-based zeolitecatalysts.

However, in use, ZSM-5 and Beta zeolites have a number of drawbacks.They are susceptible to dealumination during high temperaturehydrothermal ageing resulting in a loss of acidity, especially withCu/Beta and Cu/ZSM-5 catalysts. Both Beta- and ZSM-5-based catalysts arealso affected by hydrocarbons which become adsorbed on the catalysts atrelatively low temperatures and are oxidised as the temperature of thecatalytic system is raised generating a significant exotherm, which canthermally damage the catalyst. This problem is particularly acute invehicular diesel applications where significant quantities ofhydrocarbon can be adsorbed on the catalyst during cold-start; and Betaand ZSM-5 zeolites are also prone to coking by hydrocarbons.

In general, Cu-based zeolite catalysts are less thermally durable, andproduce higher levels of N₂O than Fe-based zeolite catalysts. However,they have a desirable advantage in that they slip less ammonia in usecompared with a corresponding Fe-zeolite catalyst.

It has been reported that aluminophosphate zeolites that containtransition metals demonstrate enhanced catalytic activity and superiorthermal stability than aluminosilicate zeolite catalysts for SCR of NOwith hydrocarbons (also known as lean NO catalysis or “DeNOx catalysts”(e.g. Ishihara et al., Journal of Catalysis, 169 (1997) 93)). In asimilar vein, WO 2006/064805 discloses an electrical processingtechnology for treating diesel engine exhaust gas which utilizes coronadischarge. A combination of a device for adding a NO reducer(hydrocarbon or fuel) and a Cu-SAPO-34 NO reducing catalyst can bedisposed downstream of the electrical processing apparatus. However, toour knowledge, there has been no investigation of transitionmetal-containing aluminophosphate zeolites for SCR of NO with NH₃ (orurea) reported in any literature to date.

WO 00/72965 discloses iron (Fe) exchanged zeolites for the selectivecatalytic reduction of nitrogen monoxide by ammonia for controlling NOemissions from fossil-fuel power plants and engines. The Fe-exchanged,and optionally Fe-rare earth-exchanged, e.g. Fe—Ce-exchanged, zeolitessuggested include: ZSM-5, mordenite, SAPO, clinoptilolite, chabazite,ZK-4 and ZK-5. No specific SAPO zeolites are identified and noexperiment using SAPO zeolites is disclosed. Moreover, WO '965 teachesthat the disclosure has application to zeolites with a range of poresizes, i.e. large (mordenite), medium (ZSM-5, clinoptilolite) and small(chabazite, ZK-4, ZK-5) pore zeolites, with Fe-ZSM-5 preferred. There isno teaching or suggestion of any advantage in the use of small porezeolites compared with medium and large pore zeolites. Moreover, ZK-4zeolite is potentially hydrothermally unstable.

U.S. Pat. No. 4,735,927 discloses an extruded-type NH₃-SCR catalyst withstability to sulfur poisoning comprising a high surface area titania inthe form of anatase and a natural or synthetic zeolite. The zeolite mustbe either in the acid form or thermally convertible to the acid form inthe catalytic product. Examples of suitable zeolites include mordenite,natural clinoptilolite, erionite, heulandite, ferrierite, naturalfaujasite or its synthetic counterpart zeolite Y, chabazite andgmelinite. A preferred zeolite is natural clinoptilolite, which may bemixed with another acid stable zeolite such as chabazite. The catalystmay optionally include small amounts (at least 0.1% by elemental weight)of a promoter in the form of precursors of vanadium oxide, copper oxide,molybdenum oxide or combinations thereof (0.2 wt % Cu and up to 1.6 wt %V are exemplified). Extruded-type catalysts are generally less durable,have lower chemical strength, require more catalyst material to achievethe same activity and are more complicated to manufacture than catalystcoatings applied to inert monolith substrates.

U.S. Pat. No. 5,417,949 also discloses an extruded-type NH₃-SCR catalystcomprising a zeolite having a constraint index of up to 12 and a titaniabinder. Intentionally, no transition metal promoter is present.(“Constraint Index” is a test to determine shape-selective catalyticbehaviour in zeolites. It compares the reaction rates for the crackingof n-hexane and its isomer 3-methylpentane under competitive conditions(see V. J. Frillette et al., J Catal. 67 (1991) 218)).

U.S. Pat. No. 5,589,147 discloses an ammonia SCR catalyst comprising amolecular sieve and a metal, which catalyst can be coated on a substratemonolith. The molecular sieve useful in the invention is not limited toany particular molecular sieve material and, in general, includes allmetallosilicates, metallophosphates, silicoaluminophosphates and layeredand pillared layered materials. The metal is typically selected from atleast one of the metals of Groups of the Periodic Table IIIA, IB, IIB,VA, VIA, VIIA, VIIIA and combinations thereof. Examples of these metalsinclude at least one of copper, zinc, vanadium, chromium, manganese,cobalt, iron, nickel, rhodium, palladium, platinum, molybdenum,tungsten, cerium and mixtures thereof.

The disclosure of U.S. Pat. No. 5,589,147 is ambiguous about whethersmall pore zeolites (as defined herein) have any application in theprocess of the invention. For example, on the one hand, certain smallpore zeolites are mentioned as possible zeolites for use in theinvention, i.e. erionite and chabazite, while, among others, themolecular sieve SAPO-34 was “contemplated”. On the other hand a table ispresented listing Constraint Index (CI) values for some typical zeolites“including some which are suitable as catalysts in the process of thisinvention”. The vast majority of the CI values in the table are wellbelow 10, of which erionite (38 at 316° C.) and ZSM-34 (50 at 371° C.)are notable exceptions. However, what is clear is that intermediate poresize zeolites, e.g. those having pore sizes of from about 5 to less than7 Angstroms, are preferred in the process of the invention. Inparticular, the disclosure explains that intermediate pore size zeolitesare preferred because they provide constrained access to and egress fromthe intracrystalline free space: “The intermediate pore size zeolites .. . have an effective pore size such as to freely sorb normal hexane . .. if the only pore windows in a crystal are formed by 8-membered ringsof oxygen atoms, then access to molecules of larger cross-section thannormal hexane is excluded and the zeolite is not an intermediate poresize material.” Only extruded Fe-ZSM-5 is exemplified.

WO 2004/002611 discloses an NH₃-SCR catalyst comprising a ceria-dopedaluminosilicate zeolite.

U.S. Pat. No. 6,514,470 discloses a process for catalytically reducingNO in an exhaust gas stream containing nitrogen oxides and a reductantmaterial. The catalyst comprises an aluminium-silicate material and ametal in an amount of up to about 0.1 weight percent based on the totalweight of catalyst. All of the examples use ferrierite.

Long et al. Journal of Catalysis 207 (2002) 274-285 reports on studiesof Fe³⁺-exchanged zeolites for selective catalytic reduction of NO withammonia. The zeolites investigated were mordenite, clinoptilolite, Beta,ferrierite and chabazite. It was found that in the conditions studiedthat the SCR activity decreases in the following order:Fe-mordenite>Fe-clinoptilolite>Fe-ferrierite>Fe-Beta>Fe-chabazite. Thechabazite used for making the Fe-chabazite was a naturally occurringmineral.

U.S. Pat. No. 4,961,917 discloses an NH₃-SCR catalyst comprising azeolite having a silica-to-alumina ratio of at least about 10, and apore structure which is interconnected in all three crystallographicdimensions by pores having an average kinetic pore diameter of at leastabout 7 Angstroms and a Cu or Fe promoter. The catalysts are said tohave high activity, reduced NH₃ oxidation and reduced sulphur poisoning.Zeolite Beta and zeolite Y are two zeolites that meet the requireddefinition.

U.S. Pat. No. 3,895,094 discloses an NH₃-SCR process using zeolitecatalysts of at least 6 Angstrom intercrystalline pore size. No mentionis made of exchanging the zeolites with transition metals.

U.S. Pat. No. 4,220,632 also discloses an NH₃-SCR process, this timeusing 3-10 Angstrom pore size zeolites of Na or H form.

WO 02/41991 discloses metal promoted zeolite Beta for NH₃-SCR, whereinthe zeolite is pre-treated so as to provide it with improvedhydrothermal stability.

There is a need in the art for SCR catalysts that have relatively goodlow temperature SCR activity, that have relatively high selectivity toN₂—in particular low N₂O formation, that have relatively good thermaldurability and are relatively resistant to hydrocarbon inhibition. Wehave now discovered a family of transition metal-containing zeolitesthat meet or contribute to this need.

According to one aspect, the invention provides a method of convertingnitrogen oxides in a gas to nitrogen by contacting the nitrogen oxideswith a nitrogenous reducing agent in the presence of a zeolite catalystcontaining at least one transition metal, wherein the zeolite is a smallpore zeolite containing a maximum ring size of eight tetrahedral atoms,wherein the at least one transition metal is selected from the groupconsisting of Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag,In, Sn, Re, Ir and Pt.

By “zeolite catalyst containing at least one transition metal” herein wemean a zeolite structure to which has been added by ion exchange,impregnation or isomorphous substitution etc. one or more metals.“Transition metal-containing zeolite catalyst” and “zeolite catalystcontaining at least one transition metal” and similar terms are usedinterchangeably herein.

It will be appreciated that by defining the zeolites by their FrameworkType Codes we intend to include the “Type Material” and any and allisotypic framework materials. (The “Type Material” is the species firstused to establish the framework type). Reference is made to Table 1,which lists a range of illustrative zeolite zeotype framework materialsfor use in the present invention. For the avoidance of doubt, unlessotherwise made clear, reference herein to a zeolite by name, e.g.“chabazite”, is to the zeolite material per se (in this example thenaturally occurring type material chabazite) and not to any othermaterial designated by the Framework Type Code to which the individualzeolite may belong, e.g. some other isotypic framework material. So forexample, where the attached claims disclaim a zeolite catalyst, thisdisclaimer should be interpreted narrowly, so that “wherein thetransition metal-containing small pore zeolite is not Cu/chabazite” isintended to exclude the type material and not any isotypic frameworkmaterials such as SAPO-34 or SSZ-13. Equally, use of a FTC herein isintended to refer to the Type Material and all isotypic frameworkmaterials defined by that FTC. For further information, we direct thereader to the website of the International Zeolite Association atwww.iza-online.org.

The distinction between zeolite type materials, such as naturallyoccurring (i.e. mineral) chabazite, and isotypes within the sameFramework Type Code is not merely arbitrary, but reflects differences inthe properties between the materials, which may in turn lead todifferences in activity in the method of the present invention. Forexample, in addition to the comments made hereinbelow with reference toLong et al. Journal of Catalysis 207 (2002) 274-285, the naturallyoccurring chabazite has a lower silica-to-alumina ratio thanaluminosilicate isotypes such as SSZ-13, the naturally occurringchabazite has lower acidity than aluminosilicate isotypes such as SSZ-13and the activity of the material in the method of the present inventionis relatively low (see the comparison of Cu/naturally occurringchabazite with Cu/SAPO-34 in Example 13).

The zeolite catalysts for use in the present invention can be coated ona suitable substrate monolith or can be formed as extruded-typecatalysts, but are preferably used in a catalyst coating.

Whilst the prior art (such as the documents discussed in the backgroundsection hereinabove) does mention a few small pore zeolites containingat least one transition metal for converting nitrogen oxides in a gas tonitrogen with a nitrogenous reducing agent, there is no appreciation inthe prior art that we can find of the particular advantages of usingsmall pore zeolites containing at least one transition metal for thispurpose. Thus, the prior art suggests using large, medium and small porezeolites containing at least one transition metal, without distinction.Accordingly, we seek to exclude any specific small pore zeolitescontaining at least one transition metal that have been mentioned onlyin this context.

In this regard, in one embodiment, the zeolite catalyst is not one ofCo, Ga, Mn, In or Zn or any combination of two or morethereof/epistilbite (see U.S. Pat. No. 6,514,470). In anotherembodiment, the transition metal-containing small pore zeolite is notCu/chabazite, Mo/chabazite, Cu—Mo/chabazite, Cu/erionite, Mo/erionite orCu—Mo/erionite (see U.S. Pat. No. 4,735,927). In a further embodiment,the transition metal-containing small pore zeolite is not Ce/erionite(see WO 2004/002611). In a further embodiment, the transitionmetal-containing small pore zeolite is not Fe/chabazite, Fe/ZK-5,Fe/ZK-4, Fe-rare-earth/chabazite, Fe-rare-earth/ZK-5 orFe-rare-earth/ZK-4 (see WO 00/72965). Furthermore, although WO 00/72965discloses the use of Ce/SAPO zeolites and Ce-rare-earth/SAPO zeolites ingeneral, it does not disclose any particular small pore SAPO zeoliteswith application in the present invention, such as SAPO-17, SAPO-18,SAPO-34, SAPO-35, SAPO-39, SAPO-43 and SAPO-56. In yet a furtherembodiment, the transition metal-containing small pore zeolite is notFe/chabazite, (see Long et al. Journal of Catalysis 207 (2002) 274-285).Whilst, for the reasons given hereinabove, we do not believe that U.S.Pat. No. 5,589,147 discloses the use of small pore zeolites containingat least one transition metal according the method of the presentinvention, for safety, according to another embodiment, the zeolitecatalyst is not any one of copper, zinc, chromium, manganese, cobalt,iron, nickel, rhodium, palladium, platinum, molybdenum, cerium ormixtures thereof/any one of aluminosilicate chabazite, aluminosilicateerionite, aluminosilicate ZSM-34 and SAPO-34. In another embodiment, thetransition metal-containing zeolite catalyst is not LTA or Fe/CHA.

It will be appreciated that chabazite is a small pore zeolite accordingto the definition adopted herein and that the Long et al. papermentioned above reports that Fe/chabazite has the poorest activity ofany of the catalysts tested. Without wishing to be bound by any theory,we believe that the poor performance of the Fe/chabazite in this studyis due to two principal reasons. Firstly, natural chabazite can containbasic metal cations including potassium, sodium, strontium and calcium.To obtain an active material the basic metal cations need to beexchanged for e.g. iron cations because basic metals are a known poisonof zeolite acid sites. In the reported study the natural mineral isfirst treated with NH₄Cl solution in an attempt to “flush out” theexisting cations. However, we believe that one explanation for the poorreported activity is that the acidic sites in the chabazite of thisstudy remain poisoned by basic metal cations.

Secondly, iron ions can form metal complexes (coordination compounds)with suitable ligands in the ionic exchange medium. In this regard wenote that Long et al. use an aqueous FeCl₂ solution for ion exchange.Since the zeolite pores are relatively small, it is possible that abulky co-ordination compound may not be able to gain access to theactive sites located in the pores.

It will be appreciated, e.g. from Table 1 hereinbelow that by “MeAPSO”and “MeAlPO” we intend zeotypes substituted with one or more metals.Suitable substituent metals include one or more of, without limitation,As, B, Be, Co, Fe, Ga, Ge, Li, Mg, Mn, Zn and Zr.

In a particular embodiment, the small pore zeolites for use in thepresent invention can be selected from the group consisting ofaluminosilicate zeolites, metal-substituted aluminosilicate zeolites andaluminophosphate zeolites.

Aluminophosphate zeolites with application in the present inventioninclude aluminophosphate (AlPO) zeolites, metal substituted zeolites(MeAlPO) zeolites, silico-aluminophosphate (SAPO) zeolites and metalsubstituted silico-aluminophosphate (MeAPSO) zeolites.

It will be appreciated that the invention extends to catalyst coatingsand extruded-type substrate monoliths comprising both transitionmetal-containing small pore zeolites according to the invention andnon-small pore zeolites (whether metallised or not) such as medium-,large- and meso-pore zeolites (whether containing transition metal(s) ornot) because such a combination also obtains the advantages of usingsmall pore zeolites per se. It should also be understood that thecatalyst coatings and extruded-type substrate monoliths for use in theinvention can comprise combinations of two or more transitionmetal-containing small pore zeolites. Furthermore, each small porezeolite in such a combination can contain one or more transition metals,each being selected from the group defined hereinabove, e.g. a firstsmall pore zeolite can contain both Cu and Fe and a second small porezeolite in combination with the first small pore zeolite can contain Ce.

In this invention, we have discovered that transition metal-containingsmall pore zeolites are advantageous catalysts for SCR of NO with NH₃.Compared to transition metal-containing medium, large or meso-porezeolite catalysts, transition metal-containing small pore zeolitecatalysts demonstrate significantly improved NO reduction activity,especially at low temperatures. They also exhibit high selectivity to N₂(e.g. low N₂O formation) and good hydrothermal stability. Furthermore,small pore zeolites containing at least one transition metal are moreresistant to hydrocarbon inhibition than larger pore zeolites, e.g. amedium pore zeolite (a zeolite containing a maximum ring size of 10)such as ZSM-5 or a large pore zeolite (a zeolite having a maximum ringsize of 12), such as Beta. Small pore aluminophosphate zeolites for usein the present invention include SAPO-17, SAPO-18, SAPO-34, SAPO-35,SAPO-39, SAPO-43 and SAPO-56.

In one embodiment, the small pore zeolite is selected from the group ofFramework Type Codes consisting of: ACO, AEI, AEN, AFN, AFT, AFX, ANA,APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS, GOO, IHW,ITE, ITW, LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV,SIV, THO, TSC, UEI, UFI, VNI, YUG and ZON.

Zeolites with application in the present invention can include thosethat have been treated to improve hydrothermal stability. Illustrativemethods of improving hydrothermal stability include: (i) Dealuminationby: steaming and acid extraction using an acid or complexing agent e.g.(EDTA—ethylenediaminetetracetic acid); treatment with acid and/orcomplexing agent; treatment with a gaseous stream of SiCl₄ (replaces Alin the zeolite framework with Si); (ii) Cation exchange—use ofmulti-valent cations such as La; and (iii) Use of phosphorous containingcompounds (see e.g. U.S. Pat. No. 5,958,818).

We believe that small pore zeolites may minimise the detrimental effectof hydrocarbons by means of a molecular sieving effect, whereby thesmall pore zeolite allows NO and NH₃ to diffuse to the active sitesinside the pores but that the diffusion of hydrocarbon molecules isrestricted. In this regard, the kinetic diameter of both NO (3.16 Å) andNH₃ (2.6 Å) is smaller than those of the typical hydrocarbons (C₃H₆˜4.5Å, n-C₈H₁₈˜4.30 Å and C₇H₈˜6.0 Å) present in, for example, diesel engineexhaust. Accordingly, in one embodiment the small pore zeolite catalystsfor use in the present invention have a pore size in at least onedimension of less than 4.3 Å. Illustrative examples of suitable smallpore zeolites are set out in Table 1.

TABLE 1 Details of small pore zeolites with application in the presentinvention Zeolite Framework Type material* and Type (by illustrativeisotypic Framework framework Dimension- Type Code) structures ality Poresize (Å) Additional info ACO *ACP-1 3D 3.5 × 2.8, 3.5 × Ring sizes - 8,4 3.5 AEI *AlPO-18 3D 3.8 × 3.8 Ring sizes - 8, 6, 4 [Co—Al—P—O]-AEISAPO-18 SIZ-8 SSZ-39 AEN *AlPO-EN3 2D 4.3 × 3.1, 2.7 × Ring sizes - 8,6, 4 5.0 AlPO-53(A) AlPO-53(B) [Ga—P—O]-AEN CFSAPO-1A CoIST-2 IST-2JDF-2 MCS-1 MnAPO-14 Mu-10 UiO-12-500 UiO-12-as AFN *AlPO-14 3D 1.9 ×4.6, 2.1 × Ring sizes - 8, 6, 4 4.9, 3.3 × 4.0 |(C₃N₂H₁₂)—|[Mn—Al—P—O]-AFN GaPO-14 AFT *AlPO-52 3D 3.8 × 3.2, 3.8 × Ring sizes - 8, 6, 4 3.6AFX *SAPO-56 3D 3.4 × 3.6 Ring sizes - 8, 6, 4 MAPSO-56, M = Co, Mn, ZrSSZ-16 ANA *Analcime 3D 4.2 × 1.6 Ring sizes - 8, 6, 4 AlPO₄-polluciteAlPO-24 Ammonioleucite [Al—Co—P—O]-ANA [Al—Si—P—O]-ANA|Cs—|[Al—Ge—O]-ANA |Cs—|[Be—Si—O]-ANA |Cs₁₆|[Cu₈Si₄₀O₉₆]- ANA|Cs—Fe|[Si—O]-ANA |Cs—Na—(H₂O)|[Ga—Si—O]- ANA [Ga—Ge—O]-ANA|K—|[B—Si—O]-ANA |K—|[Be—B—P—O]-ANA |Li—|[Li—Zn—Si—O]-ANA|Li—Na|[Al—Si—O]-ANA |Na—|[Be—B—P—O]- ANA |(NH₄)—|[Be—B—P—O]- ANA|(NH₄)—|[Zn—Ga—P—O]- ANA [Zn—As—O]-ANA Ca-D Hsianghualite Leucite Na—BPollucite Wairakite APC *AlPO-C 2D 3.7 × 3.4, 4.7 × Ring sizes - 8, 6, 42.0 AlPO-H3 CoAPO-H3 APD *AlPO-D 2D 6.0 × 2.3, 5.8 × Ring sizes - 8, 6,4 1.3 APO-CJ3 ATT *AlPO-12-TAMU 2D 4.6 × 4.2, 3.8 × Ring sizes - 8, 6, 43.8 AlPO-33 RMA-3 CDO *CDS-1 2D 4.7 × 3.1, 4.2 × Ring sizes - 8, 5 2.5MCM-65 UZM-25 CHA *Chabazite 3D 3.8 × 3.8 Ring sizes - 8, 6, 4 AlPO-34[Al—As—O]-CHA [Al—Co—P—O]-CHA |Co| [Be—P—O]-CHA |Co₃ (C₆N₄H₂₄)₃ (H₂O)₉|[Be₁₈P₁₈O₇₂]- CHA [Co—Al—P—O]-CHA |Li—Na| [Al—Si—O]- CHA [Mg—Al—P—O]-CHA[Si—O]-CHA [Zn—Al—P—O]-CHA [Zn—As—O]-CHA CoAPO-44 CoAPO-47 DAF-5 GaPO-34K-Chabazite Linde D Linde R LZ-218 MeAPO-47 MeAPSO-47 (Ni(deta)₂)-UT-6Phi SAPO-34 SAPO-47 SSZ-13 UiO-21 Willhendersonite ZK-14 ZYT-6 DDR*Deca-dodecasil 3R 2D 4.4 × 3.6 Ring sizes - 8, 6, 5, 4 [B—Si—O]-DDRSigma-1 ZSM-58 DFT *DAF-2 3D 4.1 × 4.1, 4.7 × Ring sizes - 8, 6, 4 1.8ACP-3, [Co—Al—P—O]- DFT [Fe—Zn—P—O]-DFT [Zn—Co—P—O]-DFT UCSB-3GaGeUCSB-3ZnAs UiO-20, [Mg—P—O]- DFT EAB *TMA-E 2D 5.1 × 3.7 Ring sizes - 8,6, 4 Bellbergite EDI *Edingtonite 3D 2.8 × 3.8, 3.1 × Ring sizes - 8, 42.0 |(C₃H₁₂N₂)_(2.5)| [Zn₅P₅O₂₀]-EDI [Co—Al—P—O]-EDI [Co—Ga—P—O]-EDI|Li—|[Al—Si—O]-EDI |Rb₇ Na (H₂O)₃| [Ga₈Si₁₂O₄₀]-EDI [Zn—As—O]-EDI K—FLinde F Zeolite N EPI *Epistilbite 2D 4.5 × 3.7, 3.6 × Ring sizes - 8, 43.6 ERI *Erionite 3D 3.6 × 5.1 Ring sizes - 8, 6, 4 AlPO-17 Linde TLZ-220 SAPO-17 ZSM-34 GIS *Gismondine 3D 4.5 × 3.1, 4.8 × Ring sizes -8, 4 2.8 Amicite [Al—Co—P—O]-GIS [Al—Ge—O]-GIS [Al—P—O]-GIS [Be—P—O]-GIS|(C₃H₁₂N₂)₄| [Be₈P₈O₃₂]-GIS |(C₃H₁₂N₂)₄| [Zn₈P₈O₃₂]-GIS [Co—Al—P—O]-GIS[Co—Ga—P—O]-GIS [Co—P—O]-GIS |Cs₄|[Zn₄B₄P₈O₃₂]- GIS [Ga—Si—O]-GIS[Mg—Al—P—O]-GIS |(NH₄)₄|[Zn₄B₄P₈O₃₂]- GIS |Rb₄|[Zn₄B₄P₈O₃₂]- GIS[Zn—Al—As—O]-GIS [Zn—Co—B—P—O]-GIS [Zn—Ga—As—O]-GIS [Zn—Ga—P—O]-GISGarronite Gobbinsite MAPO-43 MAPSO-43 Na-P1 Na-P2 SAPO-43 TMA-gismondineGOO *Goosecreekite 3D 2.8 × 4.0, 2.7 × Ring sizes - 8, 6, 4 4.1, 4.7 ×2.9 IHW *ITQ-32 2D 3.5 × 4.3 Ring sizes - 8, 6, 5, 4 ITE *ITQ-3 2D 4.3 ×3.8, 2.7 × Ring sizes - 8, 6, 5, 5.8 4 Mu-14 SSZ-36 ITW *ITQ-12 2D 5.4 ×2.4, 3.9 × Ring sizes - 8, 6, 5, 4.2 4 LEV *Levyne 2D 3.6 × 4.8 Ringsizes - 8, 6, 4 AlPO-35 CoDAF-4 LZ-132 NU-3 RUB-1 [B—Si—O]-LEV SAPO-35ZK-20 ZnAPO-35 KFI ZK-5 3D 3.9 × 3.9 Ring sizes - 8, 6, 4|18-crown-6|[Al—Si—O]- KFI [Zn—Ga—As—O]-KFI (Cs,K)-ZK-5 P Q MER*Merlinoite 3D 3.5 × 3.1, 3.6 × Ring sizes - 8, 4 2.7, 5.1 × 3.4, 3.3 ×3.3 [Al—Co—P—O]-MER |Ba—|[Al—Si—O]-MER |Ba—Cl—|[Al—Si—O]- MER[Ga—Al—Si—O]-MER |K—|[Al—Si—O]-MER |NH₄—|[Be—P—O]-MER K-M Linde WZeolite W MON *Montesommaite 2D 4.4 × 3.2, 3.6 × Ring sizes - 8, 5, 43.6 [Al—Ge—O]-MON NSI *Nu-6(2) 2D 2.6 × 4.5, 2.4 × Ring sizes - 8, 6, 54.8 EU-20 OWE *UiO-28 2D 4.0 × 3.5, 4.8 × Ring sizes - 8, 6, 4 3.2 ACP-2PAU *Paulingite 3D 3.6 × 3.6 Ring sizes - 8, 6, 4 [Ga—Si—O]-PAU ECR-18PHI *Phillipsite 3D 3.8 × 3.8, 3.0 × Ring sizes - 8, 4 4.3, 3.3 × 3.2[Al—Co—P—O]-PHI DAF-8 Harmotome Wellsite ZK-19 RHO *Rho 3D 3.6 × 3.6Ring sizes - 8, 6, 4 [Be—As—O]-RHO [Be—P—O]-RHO [Co—Al—P—O]-RHO|H—|[Al—Si—O]-RHO [Mg—Al—P—O]-RHO [Mn—Al—P—O]-RHO |Na₁₆Cs₈|[Al₂₄Ge₂₄O₉₆]-RHO |NH₄—|[Al—Si—O]-RHO |Rb—|[Be—As—O]-RHO GallosilicateECR-10 LZ-214 Pahasapaite RTH *RUB-13 2D 4.1 × 3.8, 5.6 × Ring sizes -8, 6, 5, 2.5 4 SSZ-36 SSZ-50 SAT *STA-2 3D 5.5 × 3.0 Ring sizes - 8, 6,4 SAV *Mg-STA-7 3D 3.8 × 3.8, 3.9 × Ring sizes - 8, 6, 4 3.9 Co-STA-7Zn-STA-7 SBN *UCSB-9 3D TBC Ring sizes - 8, 4, 3 SU-46 SIV *SIZ-7 3D 3.5× 3.9, 3.7 × Ring sizes - 8, 4 3.8, 3.8 × 3.9 THO *Thomsonite 3D 2.3 ×3.9, 4.0 × Ring sizes - 8, 4 2.2, 3.0 × 2.2 [Al—Co—P—O]-THO[Ga—Co—P—O]-THO |Rb₂₀|[Ga₂₀Ge₂₀O₈₀]- THO [Zn—Al—As—O]-THO [Zn—P—O]-THO[Ga—Si—O]-THO) [Zn—Co—P—O]-THO TSC *Tschörtnerite 3D 4.2 × 4.2, 5.6 ×Ring sizes - 8, 6, 4 3.1 UEI *Mu-18 2D 3.5 × 4.6, 3.6 × Ring sizes - 8,6, 4 2.5 UFI *UZM-5 2D 3.6 × 4.4, 3.2 × Ring sizes - 8, 6, 4 3.2 (cage)VNI *VPI-9 3D 3.5 × 3.6, 3.1 × Ring sizes - 8, 5, 4, 4.0 3 YUG*Yugawaralite 2D 2.8 × 3.6, 3.1 × Ring sizes - 8, 5, 4 5.0 Sr-Q ZON*ZAPO-M1 2D 2.5 × 5.1, 3.7 × Ring sizes - 8, 6, 4 4.4 GaPO-DAB-2 UiO-7

Small pore zeolites with particular application for treating NO inexhaust gases of lean-burn internal combustion engines, e.g. vehicularexhaust gases are set out in Table 2.

TABLE 2 Preferred small pore zeolites for use in treating exhaust gasesof lean-burn internal combustion engines. Structure Zeolite CHA SAPO-34AlPO-34 SSZ-13 LEV Levynite Nu-3 LZ-132 SAPO-35 ZK-20 ERI ErioniteZSM-34 Linde type T DDR Deca-dodecasil 3R Sigma-1 KFI ZK-5 18-crown-6[Zn—Ga—As—O]-KFI EAB TMA-E PAU ECR-18 MER Merlinoite AEI SSZ-39 GOOGoosecreekite YUG Yugawaralite GIS P1 VNI VPI-9

Small pore aluminosilicate zeolites for use in the present invention canhave a silica-to-alumina ratio (SAR) of from 2 to 300, optionally 4 to200 and preferably 8 to 150. It will be appreciated that higher SARratios are preferred to improve thermal stability but this maynegatively affect transition metal exchange. Therefore, in selectingpreferred materials consideration can be given to SAR so that a balancemay be struck between these two properties.

The gas containing the nitrogen oxides can contact the zeolite catalystat a gas hourly space velocity of from 5,000 hr⁻¹ to 500,000 hr⁻¹,optionally from 10,000 hr⁻¹ to 200,000 hr⁻¹.

In one embodiment, the small pore zeolites for use in the presentinvention do not include aluminophosphate zeolites as defined herein. Ina further embodiment, the small pore zeolites (as defined herein) foruse in the present invention are restricted to aluminophosphate zeolites(as defined herein). In a further embodiment, small pore zeolites foruse in the present invention are aluminosilicate zeolites and metalsubstituted aluminosilicate zeolites (and not aluminophosphate zeolitesas defined herein).

Small pore zeolites for use in the invention can have three-dimensionaldimensionality, i.e. a pore structure which is interconnected in allthree crystallographic dimensions, or two-dimensional dimensionality. Inone embodiment, the small pore zeolites for use in the present inventionconsist of zeolites having three-dimensional dimensionality. In anotherembodiment, the small pore zeolites for use in the present inventionconsist of zeolites having two-dimensional dimensionality.

In one embodiment, the at least one transition metal is selected fromthe group consisting of Cr, Ce, Mn, Fe, Co, Ni and Cu. In a preferredembodiment, the at least one transition metal is selected from the groupconsisting of Cu, Fe and Ce. In a particular embodiment, the at leastone transition metal consists of Cu. In another particular embodiment,the at least one transition metal consists of Fe. In a furtherparticular embodiment, the at least one transition metal is Cu and/orFe.

The total of the at least one transition metal that can be included inthe at least one transition metal-containing zeolite can be from 0.01 to20 wt %, based on the total weight of the zeolite catalyst containing atleast one transition metal. In one embodiment, the total of the at leastone transition metal that can be included can be from 0.1 to 10 wt %. Ina particular embodiment, the total of the at least one transition metalthat can be included is from 0.5 to 5 wt %.

A preferred transition metal-containing two dimensional small porezeolite for use in the present invention consists of Cu/LEV, such asCu/Nu-3, whereas a preferred transition metal-containing threedimensional small pore zeolite/aluminophosphate zeolite for use in thepresent invention consists of Cu/CHA, such as Cu/SAPO-34 or Cu/SSZ-13.In another embodiment, particularly where a ratio of NO/NO₂ is adjusted,e.g. by using a suitable oxidation catalyst (see hereinbelow) to about1:1, Fe-containing zeolite catalysts are preferred, such as Fe-CHA, e.g.Fe/SAPO-34 or Fe/SSZ-13. Preliminary analysis indicates that Cu/SSZ-13and Cu/Nu-3 are more resistant than the equivalent Cu/SAPO-34 toextended severe high temperature lean hydrothermal ageing (900° C. for 3hours in 4.5% H₂O/air mixture cf. Example 4).

The at least one transition metal can be included in the zeolite by anyfeasible method. For example, it can be added after the zeolite has beensynthesised, e.g. by incipient wetness or exchange process; or the atleast one metal can be added during zeolite synthesis.

The zeolite catalyst for use in the present invention can be coated,e.g. as a washcoat component, on a suitable monolith substrate, such asa metal or ceramic flow through monolith substrate or a filteringsubstrate, such as a wall-flow filter or sintered metal or partialfilter (such as is disclosed in WO 01/80978 or EP 1057519, the latterdocument describing a substrate comprising convoluted flow paths that atleast slows the passage of soot therethrough). Alternatively, thezeolites for use in the present invention can be synthesized directlyonto the substrate. Alternatively, the zeolite catalysts according tothe invention can be formed into an extruded-type flow through catalyst.

The small pore zeolite catalyst containing at least one transition metalfor use in the present invention is coated on a suitable monolithsubstrate. Washcoat compositions containing the zeolites for use in thepresent invention for coating onto the monolith substrate formanufacturing extruded type substrate monoliths can comprise a binderselected from the group consisting of alumina, silica, (non zeolite)silica-alumina, naturally occurring clays, TiO₂, ZrO₂, and SnO₂.

In one embodiment, the nitrogen oxides are reduced with the reducingagent at a temperature of at least 100° C. In another embodiment, thenitrogen oxides are reduced with the reducing agent at a temperaturefrom about 150° C. to 750° C. The latter embodiment is particularlyuseful for treating exhaust gases from heavy and light duty dieselengines, particularly engines comprising exhaust systems comprising(optionally catalysed) diesel particulate filters which are regeneratedactively, e.g. by injecting hydrocarbon into the exhaust system upstreamof the filter, wherein the zeolite catalyst for use in the presentinvention is located downstream of the filter.

In a particular embodiment, the temperature range is from 175 to 550° C.In another embodiment, the temperature range is from 175 to 400° C.

In another embodiment, the nitrogen oxides reduction is carried out inthe presence of oxygen. In an alternative embodiment, the nitrogenoxides reduction is carried out in the absence of oxygen.

Zeolites for use in the present application include natural andsynthetic zeolites, preferably synthetic zeolites because the zeolitescan have a more uniform: silica-to-alumina ratio (SAR), crystallitesize, crystallite morphology, and the absence of impurities (e.g.alkaline earth metals).

The source of nitrogenous reductant can be ammonia per se, hydrazine orany suitable ammonia precursor, such as urea ((NH₂)₂CO), ammoniumcarbonate, ammonium carbamate, ammonium hydrogen carbonate or ammoniumformate.

The method can be performed on a gas derived from a combustion process,such as from an internal combustion engine (whether mobile orstationary), a gas turbine and coal or oil fired power plants. Themethod may also be used to treat gas from industrial processes such asrefining, from refinery heaters and boilers, furnaces, the chemicalprocessing industry, coke ovens, municipal waste plants andincinerators, coffee roasting plants etc.

In a particular embodiment, the method is used for treating exhaust gasfrom a vehicular lean burn internal combustion engine, such as a dieselengine, a lean-burn gasoline engine or an engine powered by liquidpetroleum gas or natural gas.

According to a further aspect, the invention provides an exhaust systemfor a vehicular lean burn internal combustion engine, which systemcomprising a conduit for carrying a flowing exhaust gas, a source ofnitrogenous reductant, a zeolite catalyst containing at least onetransition metal disposed in a flow path of the exhaust gas and meansfor metering nitrogenous reductant into a flowing exhaust gas upstreamof the zeolite catalyst, wherein the zeolite catalyst is a small porezeolite containing a maximum ring size of eight tetrahedral atoms,wherein the at least one transition metal is selected from the groupconsisting of Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag,In, Sn, Re, Jr and Pt.

For the avoidance of doubt, the small pore transition metal-containingzeolites for use in the exhaust system aspect of the present inventioninclude any for use in the method according to the invention asdescribed hereinabove.

In one embodiment, the zeolite catalyst is coated on a flow-throughmonolith substrate (i.e. a honeycomb monolithic catalyst supportstructure with many small, parallel channels running axially through theentire part) or filter monolith substrate such as a wall-flow filteretc., as described hereinabove. In another embodiment, the zeolitecatalyst is formed into an extruded-type catalyst.

The system can include means, when in use, for controlling the meteringmeans so that nitrogenous reductant is metered into the flowing exhaustgas only when it is determined that the zeolite catalyst is capable ofcatalysing NO_(x) reduction at or above a desired efficiency, such as atabove 100° C., above 150° C. or above 175° C. The determination by thecontrol means can be assisted by one or more suitable sensor inputsindicative of a condition of the engine selected from the groupconsisting of: exhaust gas temperature, catalyst bed temperature,accelerator position, mass flow of exhaust gas in the system, manifoldvacuum, ignition timing, engine speed, lambda value of the exhaust gas,the quantity of fuel injected in the engine, the position of the exhaustgas recirculation (EGR) valve and thereby the amount of EGR and boostpressure.

In a particular embodiment, metering is controlled in response to thequantity of nitrogen oxides in the exhaust gas determined eitherdirectly (using a suitable NO_(x) sensor) or indirectly, such as usingpre-correlated look-up tables or maps—stored in the controlmeans—correlating any one or more of the abovementioned inputsindicative of a condition of the engine with predicted NO_(x) content ofthe exhaust gas.

The control means can comprise a pre-programmed processor such as anelectronic control unit (ECU).

The metering of the nitrogenous reductant can be arranged such that 60%to 200% of theoretical ammonia is present in exhaust gas entering theSCR catalyst calculated at 1:1 NH₃/NO and 4:3 NH₃/NO₂.

In a further embodiment, an oxidation catalyst for oxidising nitrogenmonoxide in the exhaust gas to nitrogen dioxide can be located upstreamof a point of metering the nitrogenous reductant into the exhaust gas.In one embodiment, the oxidation catalyst is adapted to yield a gasstream entering the SCR zeolite catalyst having a ratio of NO to NO₂ offrom about 4:1 to about 1:3 by volume, e.g. at an exhaust gastemperature at oxidation catalyst inlet of 250° C. to 450° C. Thisconcept is disclosed in S. Kasaoka et al. “Effect of Inlet NO/NO₂ MolarRatio and Contribution of Oxygen in the Catalytic Reduction of NitrogenOxides with Ammonia”, Nippon Kagaku Kaishi, 1978, No. 6, pp. 874-881 andWO 99/39809.

The oxidation catalyst can include at least one platinum group metal (orsome combination of these), such as platinum, palladium or rhodium,coated on a flow-through monolith substrate. In one embodiment, the atleast one platinum group metal is platinum, palladium or a combinationof both platinum and palladium. The platinum group metal can besupported on a high surface area washcoat component such as alumina, azeolite such as an aluminosilicate zeolite, silica, non-zeolite silicaalumina, ceria, zirconia, titania or a mixed or composite oxidecontaining both ceria and zirconia.

In a further embodiment, a suitable filter substrate is located betweenthe oxidation catalyst and the zeolite catalyst. Filter substrates canbe selected from any of those mentioned above, e.g. wall flow filters.Where the filter is catalysed, e.g. with an oxidation catalyst of thekind discussed above, preferably the point of metering nitrogenousreductant is located between the filter and the zeolite catalyst.Alternatively, if the filter is uncatalysed, the means for meteringnitrogenous reductant can be located between the oxidation catalyst andthe filter. It will be appreciated that this arrangement is disclosed inWO 99/39809.

In a further embodiment, the zeolite catalyst for use in the presentinvention is coated on a filter located downstream of the oxidationcatalyst. Where the filter includes the zeolite catalyst for use in thepresent invention, the point of metering the nitrogenous reductant ispreferably located between the oxidation catalyst and the filter.

In one embodiment, the control means meters nitrogenous reductant intothe flowing exhaust gas only when the exhaust gas temperature is atleast 100° C., for example only when the exhaust gas temperature is from150° C. to 750° C.

In a further aspect, there is provided a vehicular lean-burn enginecomprising an exhaust system according to the present invention.

The vehicular lean burn internal combustion engine can be a dieselengine, a lean-burn gasoline engine or an engine powered by liquidpetroleum gas or natural gas.

In order that the invention may be more fully understood, reference ismade to the following Examples by way of illustration only and withreference to the accompanying drawings, in which:

FIG. 1 is a graph showing NO conversion (at a gas hourly space velocityof 30,000 hr⁻¹) comparing transition metal-containing aluminosilicatecatalysts with a transition metal-containing aluminophosphate/small porezeolite catalyst after relatively moderate lean hydrothermal ageingperformed on a laboratory reactor;

FIG. 2 is a graph showing N₂O formation in the test shown in FIG. 1;

FIG. 3 is a graph showing NO conversion (at a gas hourly space velocityof 100,000 hr⁻¹) comparing Cu/Beta zeolite and Cu/SAPO-34 catalysts witha transition metal-containing aluminophosphate/small pore zeolitecatalyst after relatively moderate lean hydrothermal ageing performed ona laboratory reactor;

FIG. 4 is a graph showing NO conversion (at a gas hourly space velocityof 30,000 hr⁻¹) comparing transition metal-containing aluminosilicatecatalysts with a transition metal-containing aluminophosphate/small porezeolite catalyst after relatively severe lean hydrothermal ageingperformed on a laboratory reactor;

FIG. 5 is a graph showing NO conversion for fresh Cu/Zeolite catalysts;

FIG. 6 is a graph showing NO conversion for aged Cu/Zeolite catalysts;

FIG. 7 is a graph showing N₂O formation for fresh Cu/Zeolite catalystsof FIG. 5;

FIG. 8 is a graph showing N₂O formation for aged Cu/Zeolite catalysts ofFIG. 6;

FIG. 9 is a graph showing the effect of adding HC species to Cu/zeolitecatalysts during NH₃ SCR at 300° C.;

FIG. 10 is a graph showing hydrocarbon breakthrough following additionof hydrocarbon species to Cu/zeolite catalysts during NH₃ SCR at 300°C.;

FIG. 11 is a graph showing the adsorption profiles of n-octane at 150°C. flowing through the Cu zeolite catalysts;

FIG. 12 is a graph of the temperature programmed desorption (TPD) of HCspecies to Cu/zeolite catalysts after HC adsorption at 150° C.;

FIG. 13 is a graph similar to FIG. 6 comparing NO conversion activityfor aged Cu/Sigma-1, Cu-SAPO-34, Cu/SSZ-13 and Cu/Beta;

FIG. 14 is a graph similar to FIG. 8 comparing N₂O formation for theaged Cu/zeolite catalysts of FIG. 13;

FIG. 15 is a graph similar to FIG. 13 comparing NO conversion activityfor aged Cu/ZSM-34, Cu/SAPO-34, Cu/SSZ-13 and Cu/Beta catalysts;

FIG. 16 is a graph comparing the NO conversion activity of fresh andaged Cu-SAPO-34 and Cu/SSZ-13 catalysts;

FIG. 17 is a graph comparing the NO conversion activity of fresh samplesof Cu/SAPO-34 with a Cu/naturally occurring chabazite type material;

FIG. 18 is a bar chart comparing the NO conversion activity of freshCu/SAPO-34 with that of two fresh Cu/naturally occurring chabazite typematerials at two temperature data points;

FIG. 19 is a bar chart comparing the NO conversion activity of agedCu/Beta, Cu/SAPO-34, Fe/SAPO-34 and Fe/SSZ-13 catalysts at twotemperature data points;

FIG. 20 is a bar chart comparing the hydrocarbon inhibition effect ofintroducing n-octane into a feed gas for fresh Fe/Beta and Fe/SSZ-13catalysts;

FIG. 21 is a graph showing hydrocarbon breakthrough following theintroduction of n-octane in the experiment of FIG. 20;

FIG. 22 is a bar chart comparing the effect on NO conversion activityfor a fresh Fe/SSZ-13 catalyst of using 100% NO as a component of thefeed gas with using 1:1 NO:NO₂;

FIG. 23 is a schematic diagram of an embodiment of an exhaust systemaccording to the present invention.

FIG. 23 is a schematic diagram of an embodiment of an exhaust systemaccording to the present invention, wherein diesel engine 12 comprisesan exhaust system 10 according to the present invention comprising anexhaust line 14 for conveying an exhaust gas from the engine toatmosphere via tailpipe 15. In the flow path of the exhaust gas isdisposed a platinum or platinum/palladium NO oxidation catalyst 16coated on a ceramic flow-through substrate monolith. Located downstreamof oxidation catalyst 16 in the exhaust system is a ceramic wall-flowfilter 18.

An iron/small pore zeolite SCR catalyst 20 also coated on a ceramicflow-through substrate monolith is disposed downstream of the wall-flowfilter 18. An NH₃ oxidation clean-up or slip catalyst 21 is coated on adownstream end of the SCR catalyst monolith substrate. Alternatively,the NH₃ slip catalyst can be coated on a separate substrate locateddownstream of the SCR catalyst. Means (injector 22) is provided forintroducing nitrogenous reductant fluid (urea 26) from reservoir 24 intoexhaust gas carried in the exhaust line 14. Injector 22 is controlledusing valve 28, which valve is in turn controlled by electronic controlunit 30 (valve control represented by dotted line). Electronic controlunit 30 receives closed loop feedback control input from a NO sensor 32located downstream of the SCR catalyst.

In use, the oxidation catalyst 16 passively oxidises NO to NO₂,particulate matter is trapped on filter 18 and is combusted in NO₂ NOemitted from the filter is reduced on the SCR catalyst 20 in thepresence of ammonia derived from urea injected via injector 22. It isalso understood that mixtures of NO and NO₂ in the total NO content ofthe exhaust gas entering the SCR catalyst (about 1:1) are desirable forNO reduction on a SCR catalyst as they are more readily reduced toN_(2. The NH) ₃ slip catalyst 21 oxidises NH₃ that would otherwise beexhausted to atmosphere. A similar arrangement is described in WO99/39809.

EXAMPLES Example 1 Method of Making Fresh 5 wt % Fe/BetaBeta or SAPO-34or 3 wt % SSZ-13 Zeolite Catalyst

Commercially available Beta zeolite, SAPO-34 or SSZ-13 was NH₄ ⁺ ionexchanged in a solution of NH₄NO₃, then filtered. The resulting materialwas added to an aqueous solution of Fe(NO₃₎ ₃ with stiffing. The slurrywas filtered, then washed and dried. The procedure can be repeated toachieve a desired metal loading. The final product was calcined.

Example 2 Method of Making Fresh 3 wt % Cu/Aeolites

Commercially available SAPO-34, SSZ-13, Sigma-1, ZSM-34, Nu-3, ZSM-5 andBeta zeolites were NH₄ ⁺ ion exchanged in a solution of NH₄NO₃, thenfiltered. The resulting materials were added to an aqueous solution ofCu(NO₃)₂ with stiffing. The slurry was filtered, then washed and dried.The procedure can be repeated to achieve a desired metal loading. Thefinal product was calcined.

Example 3 Lean Hydrothermal Ageing

The catalysts obtained by means of Examples 1 and 2 were leanhydrothermally aged at 750° C. for 24 hours in 4.5% H₂O/air mixture.

Example 4 Severe Lean Hydrothermal Ageing

The catalysts obtained by means of Examples 1 and 2 were severely leanhydrothermally aged at 900° C. for 1 hour in 4.5% H₂O/air mixture.

Example 5 Extended Severe Lean Hydrothermal Ageing

The catalysts obtained by means of Examples 1 and 2 were severely leanhydrothermally aged at 900° C. for a period of 3 hours in 4.5% H₂O/airmixture.

Example 6 Test Conditions

Separate samples of Fe/BetaBeta prepared according to Example 1 andCu/BetaBeta, Cu/ZSM-5 and Cu/SAPO-34 prepared according to Example 2were aged according to Examples 3 and 4 and tested in a laboratoryapparatus using the following gas mixture: 350 ppm NO, 350 ppm NH₃, 14%O₂, 4.5% H₂O, 4.5% CO₂, N₂ balance. The results are shown in FIGS. 1 to4 inclusive.

Tests were also conducted on Cu/BetaBeta, Cu/ZSM-5, Cu/SAPO-34 andCu/Nu-3 prepared according to Example 2 and aged according to Example 3and tested in a laboratory apparatus using the same gas mixture asdescribed above, except in that 12% O₂ was used. The results are shownin FIGS. 5 to 8 inclusive.

Example 7 n-Octane Adsorption Test Conditions

With the catalyst loaded in a laboratory apparatus, 1000 ppm (as Clequivalents) propene, n-octane or toluene was injected during NH₃ SCR at300° C. (350 ppm NO, 350 ppm NH₃, 12% O₂, 4.5% H₂O, 4.5% CO₂, balanceN₂). Hydrocarbon desorption was measured by ramping the temperature at10° C./minute in 12% O₂, 4.5% H₂O, 4.5% CO₂, balance N₂.

Example 8 Results for Experiments Shown in FIGS. 1 to 4 Inclusive

FIG. 1 compares the NO reduction efficiencies of a Cu/SAPO-34 catalystagainst a series of aluminosilicate zeolite supported transition metalcatalysts (Cu/ZSM-5, Cu/Beta and Fe/Beta) after a mild aging. The resultclearly demonstrates that Cu/SAPO-34 has improved low temperatureactivity for SCR of NO with NH₃.

FIG. 2 compares the N₂O formation over the catalysts. It is clear thatthe Cu/SAPO-34 catalyst produced lower levels of N₂O compared to theother two Cu-containing catalysts. The Fe-containing catalyst alsoexhibits low N₂O formation, but as shown in FIG. 1, the Fe catalyst isless active at lower temperatures.

FIG. 3 compares the NO reduction efficiencies of a Cu/SAPO-34 catalystagainst a Cu/Beta catalyst tested at a higher gas hourly space velocity.The Cu/SAPO-34 catalyst is significantly more active than the Cu-Betacatalyst at low reaction temperatures.

FIG. 4 shows the NO reduction efficiencies of a Cu/SAPO-34 catalyst anda series of aluminosilicate zeolite supported transition metal catalysts(Cu/ZSM-5, Cu/Beta, and Fe/Beta) after severe lean hydrothermal aging.The result clearly demonstrates that the Cu/SAPO-34 catalyst hassuperior hydrothermal stability.

Example 9 Results for Experiments Shown in FIGS. 5 to 12 Inclusive

NH₃ SCR activity of fresh (i.e. un-aged) Cu supported on the small porezeolites SAPO-34 and Nu-3 was compared to that of Cu supported on largerpore zeolites in FIG. 5. The corresponding activity for the samecatalysts aged under severe lean hydrothermal conditions is shown inFIG. 6. Comparison of the fresh and aged activity profiles demonstratesthat hydrothermal stability is only achieved for aluminosilicatezeolites when the Cu is supported on a small pore zeolite.

The N₂O formation measured for the fresh and aged catalysts is shown inFIGS. 7 and 8, respectively. The results clearly show that N₂O formationis significantly reduced by means of supporting Cu on zeolites that donot have large pores.

FIG. 9 compares the effect of HC on Cu/zeolite catalysts where SAPO-34and Nu-3 are used as examples of small pore zeolite materials. Forcomparison, ZSM-5 and Beta zeolite are used as examples of a medium andlarge pore zeolite, respectively. Samples were exposed to different HCspecies (propene, n-octane and toluene) during NH₃ SCR reaction at 300°C. FIG. 10 shows the corresponding HC breakthrough following HCaddition.

FIG. 11 shows the adsorption profiles of n-octane at 150° C. flowingthrough different Cu/zeolite catalysts. HC breakthrough is observedalmost immediately with Cu supported on the small pore zeolites SAPO-34and Nu-3, whereas significant HC uptake is observed with Cu on Betazeolite and ZSM-5.FIG. 12 shows the subsequent HC desorption profile asa function of increasing temperature and confirms that large amounts ofHC are stored when Cu is supported on the larger pore zeolites, whereasvery little HC is stored when small pore zeolites are employed.

Example 10 Results for Experiments Shown in FIGS. 13 and 14

Cu/SSZ-13, Cu/SAPO-34, Cu/Sigma-1 and Cu/Beta prepared according toExample 2 were aged in the manner described in Example 4 and testedaccording to Example 6. The results are shown in FIG. 13, from which itcan be seen that the NO conversion activity of each of the severely leanhydrothermally aged Cu/SSZ-13, Cu/SAPO-34 and Cu/Sigma-1 samples issignificantly better than that of the corresponding large-pore zeolite,Cu/Beta. Moreover, from FIG. 14 it can be seen that Cu/Beta generatessignificantly more N₂O than the Cu/small-pore zeolite catalysts.

Example 11 Results for Experiments Shown in FIG. 15

Cu/ZSM-34, Cu/SAPO-34, Cu/SSZ-13 and Cu/Beta prepared according toExample 2 were aged in the manner described in Example 3 and testedaccording to Example 6. The results are shown in FIG. 15, from which itcan be seen that the NO conversion activity of each of the leanhydrothermally aged Cu/SSZ-13, Cu/SAPO-34 and Cu/ZSM-34 samples issignificantly better than that of the corresponding large-pore zeolite,Cu/Beta.

Example 12 Results for Experiments Shown in FIG. 16

Fresh samples of Cu/SSZ-13 and Cu/SAPO-34 were prepared according toExample 2, samples of which were aged in the manner described in Example5. Fresh (i.e. un-aged) and aged samples were tested according toExample 6 and the results are shown in FIG. 16, from which it can beseen that the NO conversion activity of Cu/SSZ-13 is maintained evenafter extended severe lean hydrothermal ageing.

Example 13 Results for Experiments Shown in FIGS. 17 and 18

Cu/SAPO-34 and a Cu/naturally occurring chabazite type material having aSAR of about 4 were prepared according to Example 2 and the freshmaterials were tested according to Example 6. The results are shown inFIG. 17, from which it can be seen that the NO conversion activity ofthe naturally occurring Cu/chabazite is significantly lower thanCu/SAPO-34. FIG. 18 is a bar chart comparing the NO conversion activityof two fresh Cu/naturally occurring chabazite type materials preparedaccording to Example 2 at two temperature data points (200° C. and 300°C.), a first chabazite material having a SAR of about 4 and a secondchabazite material of SAR about 7. It can be seen that whilst the NOconversion activity for the SAR 7 chabazite is better than for the SAR 4chabazite material, the activity of the SAR 7 chabazite material isstill significantly lower than the fresh Cu/SAPO-34.

Example 14 Results for Experiments Shown in FIG. 19

Cu/SAPO-34 and Cu/Beta were prepared according to Example 2. Fe/SAPO-34and Fe/SSZ-13 were prepared according to Example 1. The samples wereaged according to Example 4 and the aged samples were tested accordingto Example 6. The NO activity at the 350° C. and 450° C. data points isshown in FIG. 19, from which it can be seen that the Cu/SAPO-34,Fe/SAPO-34 and Fe/SSZ-13 samples exhibit comparable or betterperformance than the Cu/Beta reference.

Example 15 Results for Experiments Shown in FIGS. 20 and 21

Fe/SSZ-13 and Fe/Beta prepared according to Example 1 were tested freshas described in Example 7, wherein n-octane (to replicate the effects ofunburned diesel fuel in a exhaust gas) was introduced at 8 minutes intothe test. The results shown in FIG. 20 compare the NOx conversionactivity at 8 minutes into the test, but before n-octane was introducedinto the feed gas (HC−) and 8 minutes after n-octane was introduced intothe feed gas (HC+). It can be seen that the Fe/Beta activitydramatically reduces following n-octane introduction compared withFe/SSZ-13. We believe that this effect results from coking of thecatalyst.

The hypothesis that coking of the Fe/Beta catalyst is responsible forthe dramatic reduction of NO conversion activity is reinforced by theresults shown in FIG. 21, wherein Cl hydrocarbon is detected downstreamof the Fe/SSZ-13 catalyst almost immediately after n-octane isintroduced into the feed gas at 8 minutes. By comparison, asignificantly lower quantity of Cl hydrocarbon is observed in theeffluent for the Fe/Beta sample. Since there is significantly less Clhydrocarbon present in the effluent for the Fe/Beta sample, and then-octane must have gone somewhere, the results suggest that it hasbecome coked on the Fe/Beta catalyst, contributing to the loss in NO_(x)conversion activity.

Example 16 Results for Experiments Shown in FIG. 22

Fe/SSZ-13 prepared according to Example 1 was tested fresh, i.e. withoutageing, in the manner described in Example 6. The test was then repeatedusing identical conditions, except in that the 350 ppm NO was replacedwith a mixture of 175 ppm NO and 175 ppm NO₂, i.e. 350 ppm total NO_(x).The results from both tests are shown in FIG. 22, from which thesignificant improvement obtainable from increasing the NO₂ content of NOin the feed gas to 1:1 can be seen. In practice, the NO:NO₂ ratio can beadjusted by oxidising NO in an exhaust gas, e.g. of a diesel engine,using a suitable oxidation catalyst located upstream of the NH₃-SCRcatalyst.

1. A catalyst for treating exhaust gas comprising at least one smallpore molecular sieve having an KFI framework and containing from 0.01 to20 weight percent of a metal selected from the group consisting of Cr,Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag, In, Sn, Re, Jr andPt and combinations thereof, based on the total weight of the molecularsieve.
 2. The catalyst of claim 1, wherein said molecular sieve hassilica-to-alumina ratio (SAR) of 2 to
 300. 3. The catalyst of claim 1,wherein said molecular sieve has an isotype selected from ZK-5,118-crown-61[Al—Si—O]-KFI, [Zn—Ga,As—O]-KFI, (Cs,K)-ZK-5, P, and Qisotypes.
 4. The catalyst of claim 1, wherein said molecular sieve is aZK-5 isotype.
 5. The catalyst of claim 1, wherein said metal is copper.6. The catalyst of claim 1, wherein said metal is platinum.
 7. Thecatalyst of claim 1, wherein said molecular sieve is an aluminosilicateand has an SAR of 8 to
 150. 8. The catalyst of claim 1, wherein saidtransition metal is present in an amount of 0.5 to 5 wt. % based on thetotal weight of the molecular sieve.
 9. The catalyst of claim 1, whereinsaid transition metal is added to the zeolite by one of incipientwetness, ion exchange, and during zeolite synthesis.
 10. The catalyst ofclaim 1, further comprising a metal promoted medium, large, or meso-poremolecular sieve.
 11. The catalyst of claim 1, further comprising a Cu orFe promoted molecular sieve having a chabazite framework.
 12. Acatalytic washcoat comprising a catalyst according to any of claims 1,and further comprising at least one binder selected from alumina,silica, non-zeolite silica-alumina, natural clay, TiO₂, ZrO₂, and SnO₂.13. A catalytic article comprising a washcoat according to claim 12coated on a substrate selected from a metal flow-through substrate, aceramic flow-through substrate, a wall-flow filter, a sintered metalfilter, and a partial filter.
 14. A catalytic article comprising anextruded-type flow through honeycomb formed from an extrudate comprisinga catalyst according to claim
 1. 15. The catalytic article of claim 14further comprising a transition metal promoted zeolite coating.
 16. Asystem for treating exhaust gas comprising a selective catalyticreduction (SCR) catalyst upstream of an NH3 oxidation catalyst (AMOX),wherein in at least one of said SCR and AMOX catalysts comprise acatalyst according to claim
 1. 17. The system of claim 16 furthercomprising a catalyzed soot filter and a diesel oxidation catalysthaving a platinum group metal, wherein said diesel oxidation catalyst isdisposed upstream of said selective catalytic reduction catalyst.
 18. Amethod for treating an exhaust gas comprising the steps of: a.contacting an exhaust gas stream containing NOx and/or NH₃ in thepresence of a catalyst according to claim 1; and b. converting at leasta portion of said NO to N₂ and/or converting at least a portion of NH₃to at least one of N₂ and NO₂.
 19. The method of claim 18, wherein saidexhaust gas contains NOx, a nitrogenous reducing agent, and a ratio ofNO to NO₂ from about 4:1 to about 1:3 by volume, and said catalyst is aselective reduction catalyst.
 20. The method of claim 18, wherein saidexhaust gas contains NH₃ and said catalyst is an ammonia slip catalyst.